As is well known, the range of a radar echo producing object can be determined by the elapsed time between propagation of a radar signal and receipt of an echo. The distance travelled by the radar signal, twice the range of the echo producing object is determined by multiplying C, the speed of light, by the elapsed time. A radar beam, with no beam spread, will provide a unique identification of the location of the echo producing object by the elapsed time and by the beam direction. However, radar beams have a beam width in both azimuth and elevation and unless the echo signal is time differentiated, an object which produced the echo may be located anywhere within the footprint of the beam. Time differentiation may be effected in many ways. The simplest, a single pulse, will indicate, by the elapsed time to detection of the echo, that an echo producing object lies on an annulus of radius centred on the radar platform wherefrom the radar signal is propagated but of arc length extending from edge to edge of the beam footprint. If the pulse is repeated, an ambiguity function arises in that a received echo may be caused by reflection of the first pulse from a more distant object or the reflection of the second pulse from a nearer object. The ambiguity function is related to the pulse repetition rate.
If the radar signal is propagated as a continuous wave, then it is conventional to modulate a radar frequency carrier wave with an identifiable modulation to form the radar signal. The identifiable modulation may take the form of a code. The elapsed time to receipt of the code is indicative of a range cell wherein the reflecting object lies. Repetition of the code introduces an ambiguity function which is related to the code repetition rate and, again, each range cell is part annular in shape. Thus, the ambiguity function defines range bands, each band comprising a plurality of range cells. An individual range cell may be identified by correlation as hereinafter described. If the range band is wide enough (e.g. equal to the distance between minimum and maximum desired ranges) then there will only be one correlating range cell and the location of the echo producing object (as far as range is concerned) is uniquely identified. If the range band is narrow, that is much less than the desired search range. Correlation by code will indicate that an echo producing object lies within a particular range cell of a range band, e.g. the nth cell, but not the range band in which that cell lies. Other means are then necessary to resolve the ambiguity function to determine in which range band the range cell lies. Using an n-bit code, n range cells can be identified within a range band, each range band being of width equal to the distance travelled by the radar signal during the propagation period p of the n-bit code and each range cell is of radial width equal to the distance travelled by the radar signal during each bit period that is during the time .sup.p/.sub.n seconds.
Correlation is effected by comparing the modulation of a received echo with the modulation applied to the propagated signal delayed by a sequence of predetermined intervals, the predetermined intervals being of duration .sup.p/.sub.n. Thus, the propagated signal may be tapped and delayed by a preset delay and fed to a correlator whereto an amplified received echo is also fed.
Positive correlation indicates that a radar signal reflecting object lies within that range cell corresponding to the preset delay. The range cell is of course one of a plurality of range cells repeated at the ambiguity function distance i.e. the reflecting object lies in one of the repetitive range cells associated with the successful correlation. The interval between the preset delays is arranged to be equal to the bit length of the code (.sup.p/.sub.n). As aforesaid, such an interval determines the radial width of each range cell. The code length p provides a minimum required integration period of the apparatus.
Both analogue and digital correlators are known. Analogue correlation is used in Signature Measurement and Analysis (SMA) high range resolution radars. Analogue correlation has the advantage of ability to correlate wide bandwidth (for example, 1 GHz and above) signals. It has the disadvantage of requiring, for each range cell, a separate correlator. Thus, the hardware requirement may be described as massive. Further, the data gathering potential of analogue correlators is slow when compared with digital correlation (see below).
For example, at a modulation frequency of 1 GHz, each range cell (the range resolution) is only 0.15 meters in radial width. This is considerably better than is normally required. Using a 64 bit modulation code (reasonable for analogue correlation), the ambiguity function is, however, only 9.6 meters. A longer code would increase the ambiguity function but other measures are normally resorted to to resolve the ambiguity function.
FIG. 1 is a diagrammatic representation of analogue correlation. A carrier wave 1 of radar frequency is biphase modulated using a 64 bit maximal linear code 2 with a bit rate of 1 GHz. The received signals (echoes) are fed to an analogue correlator together with the 64 bit code to which is applied a stepwise variable time delay. For each time delay T.sub.R +t.sub.n (where T.sub.R is a delay requisite to a minimum desired range and t.sub.n is the additional stepwise increment thereto between t=0 at n=1 to t=64 nano secsonds at n=64), the correlator mixes the applied signals. The output of the correlator includes an integrator or low pass filter 26. The output of the filter 26 is graphically illustrated at the bottom of FIG. 1 on the assumption that an echo occured when code input to the correlator was delayed by T.sub.R +t.sub.1 i.e. T.sub.R seconds. As stated above, the code input is delayed by a further nanosecond and fed to the correlator 22 again (or to a second similarly arranged analogue correlator) to determine if any signal reflecting object lies in the next repetitive 0.15 m range cells corresponding to a delay of T.sub.R +t.sub.2. Thus, the analogue correlator 22 is capable of reporting a reflecting object lying within a 0.15 m range cell but ambiguously. Other means are necessary to resolve the ambiguity. For each range cell in the range bands, a separate correlator can be used.
Digital correlation (known also as "compression" correlation) of digitally encoded propagated waveforms provides large quantities of range information very rapidly. However, digital correlation is limited in bandwidth (having a typical maximum between 10 MHz and 20 MHz) and its suitability in radar use is limited by the consequent poor range resolution and Doppler sidelobe performance.
For example, at a code bit rate of 10 MHz, the range resolution (the radial width of each range cell) is 15 meters. Coding the propagated wave with a 2048 bit code results in an ambiguity function of 30.72 Km. At a bit rate of 20 MHz, the resolution improves to 7.5 meters but the ambiguity function, for the same coding, drops to 15.36 Km. Whilst the higher ambiguity function is desirable, the range resolution even at 20 MHz is unsatisfactory.
It is an object of the present invention to provide a method of and an apparatus for determination of range of a radar echo producing object maximizing the advantages of both analogue and digital correlation whilst minimizing the disadvantages thereof.